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Re: DC power versus AC power

  • From: Owen DeLong
  • Date: Thu Jan 02 18:10:12 2003


Also, some AC circuit breakers are of a design that counts on the magnetic
properties of AC, and, therefore, won't trip due to ANY DC load. I think
these are mostly not available any more, but I remember encountering them
some time ago and realizing that it would be _REALLY_ bad if someone put
them in a DC plant accidentally.

Owen


--On Monday, December 30, 2002 9:18 -0500 "Robert E. Seastrom" <rs@seastrom.com> wrote:


"Barton F Bruce" <barton@gnaps.com> writes:

Typical 120/208V small branch circuit breakers in small buildings and
homes have an interrupting capacity rated at 10,000 amps, and should not
be deployed where that can be exceeded. It will be on the label.
It's worth noting that the interrupting capacity of the aforementioned
breakers is 10,000 amps *AC*, and that said circuit breakers should
not be used in *DC* applications despite the fact that the voltage is
less than half as much and the fact that they're downstream from a
600A fuse (and have smaller wire in the circuit that will naturally
limit how many amps can go into a short anyway).

I'm hazy on the theory (perhaps someone more knowledgeable can post
it), but my understanding is that with AC the arc has a chance to
quench 120 times per second (ie, every time there's a zero crossing),
and with DC that opportunity (obviously) does not exist.

Bottom line is that one should buy breakers and fuses that are
designed for use in DC powerplants, rather than trying to cheap out
with something you picked up at Home Depot or Pep Boys.  I'm sure I'm
wasting my breath since _nobody_ who reads NANOG would ever try to cut
corners to save a few bucks...  :)

                                        ---Rob







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